1. Field of the Invention
Embodiments of this invention relate generally to gas turbine cleaning methods.
More particularly, embodiment of this invention relate to methods for on-line cleaning of the internal surfaces of selected sections of a hydrocarbon fuel burning gas turbine, which may include compressor sections, hot gas paths, and when present, attendant heat recovery equipment, where the cleaning methods utilize expandable and/or non-expandable coke particles to effect surface cleaning during operation.
2. Description of the Related Art
Gas turbines burning heavy fuels in particular are subject to a rapid buildup of ash deposits on the hot gas path that requires them to be shut down on a regular basis for thorough cleaning by water washing. In these instances some ash is also deposited on downstream boiler tubes used for heat recovery when the gas turbine is operated in a co-generation mode or in combined cycle operation. This effect is still present, albeit to a lesser degree, when light distillates, natural gas, or other hydrocarbon based fuels are utilized. These deposits may result in lost power generation for the operator, amounting to potentially hundreds of thousands of dollars per month. Furthermore, deposits formed on the boiler tubes interfere with normal heat transfer; thereby reducing the quantity of steam produced which is a further drain on operating revenues.
Gas turbine technology has improved to the extent that many gas turbines can now attain nearly 60% thermal efficiencies. The efficiencies are obtainable for nearly all fuels. Many users consider heavy, dirty fuels, but regard the increased maintenance and lost power to be more troublesome than it is worth in fuel cost savings. However, the use of less than clean fuels remains only a minor percentage of fuels for this more important application. Instead, many gas turbines consume large quantities of natural gas and distillate fuels. These clean fuels may be diverted to other more important applications, if their need was reduced in gas turbine combustion.
Essentially all gas turbines, regardless of the fuel used, suffer power losses due to ingested contaminants in the large volumes of air required to support combustion. With time, these contaminants (e.g., dirt, salt spray, fuel residues, etc.) coat the extremely large surface area of the compressor section. As the deposits accumulate, compressor efficiency is affected and, in addition, the rotational speed of the turbine may be affected. It becomes necessary to introduce cleaning water and surfactants to wash off the deposits. Many times this cleaning procedure needs to await the shutdown of the gas turbine to do a thorough cleaning. In the meantime, power output and revenue are lost from the gas turbine operation.
A major problem in gas turbine operation on heavy fuels is the presence of sodium and vanadium in the heavy fuel. Both are extremely corrosive at the temperatures attained by modern gas turbines. To limit the effects of sodium, there have been strict limits on the amount of sodium in fuels. To meet the gas turbine manufacturer's requirements, fuel washing is employed. To counteract the corrosive nature of vanadium, magnesium additives must be used. Experience has shown that vanadium corrosion is inhibited when the magnesium is added at more than three times the theoretical requirement.
Ash created from the combustion of heavy fuels deposits on the gas turbine hot gas path parts. These deposits hinder the gas flow path streamline flow resulting in reduced power output, decreased efficiency (or increased heat rate), and increased compressor pressure ratio. Periodic cleaning (removal of these ash deposits) is necessary to restore lost power and efficiency, and to enable the compressor to operate within its normal limits. The rate of ash deposition is highly variable, but depends mainly on the turbine duty cycle, firing temperature (and consequently the hot gas path temperatures), and the level of fuel contaminants.
The duty cycle significantly affects the ash deposition rate. Gas turbines in peaking service that are shutdown daily may experience very slow rates of ash deposition. Some of the deposits absorb water (from the atmospheric humidity) during shutdown and spall off during restart due to thermal stresses in the deposits. This effect is reduced as the firing temperature is increased.
The hot gas path temperatures have a significant effect both on the rate of ash deposition and the type of deposit formed. Lower firing turbines will form mainly magnesium sulfate that is a soft material and will readily spall off during subsequent restarts. As firing temperatures increase above the 1700° F. to 1800° F. range, along with the hot gas path temperatures, the predominantly magnesium sulfate deposits are replaced by hard deposits composed mainly of magnesium oxide. These deposits are much harder to remove.
The fuel vanadium concentration and the attendant high level of magnesium inhibitor concentration have a significant influence on the ash deposition rate. As more ash goes through the unit per constant internal surface area, the chances become greater that this ash will build up on the hot gas path parts.
Several methods have been used over the years to clean gas turbines; the two primary methods being on-line cleaning and off-line cleaning. In on-line cleaning, nutshells (walnut, pecan, and/or rice hulls), graphite, and other substances have been introduced into the combustion chamber. Those particles that don't combust collide with the ash deposits causing them to spall off the blades. Unfortunately, many of the nutshells end up as extremely fine carbon ash that lacks the energy to clean the deposits. Also unburned nutshells can end up in bearings and other areas of the gas turbine where they are an unwanted nuisance. As a consequence of the difficulties of controlling the nutshells, this method of cleaning is used only when necessary. However, the main advantage of the nutshell technique is that it can be accomplished while the gas turbine is running (albeit usually at reduced load), generating electricity.
The other main cleaning method is off-line water washing. For this method the turbine must be taken out of service, cooled, and then water is injected through spray nozzles, while the engine is on crank speed, to thoroughly soak the deposits (some deposits may dissolve). There are many drawbacks to this method, the foremost being that the turbine is out of service during the entire cleaning operation. Water washing is used extensively to return gas turbines to full power. Additionally, the cleaning is often not adequate to completely remove all deposits. When the gas turbine is placed back into service with deposits still remaining, these deposits can become very hard from being subjected to additional periods of high temperature. When the deposits are hard enough, it may become necessary to dismantle the gas turbine to laboriously hand clean the deposits from surfaces.
There are a number of patents that disclose gas turbine cleaning methods, for cleaning both compressors and the hot gas path. Those for compressor cleaning are much more numerous than those that include the hot gas path. There are also several patents that disclose various chemical compositions used to clean deposits from gas turbine compressors, and others that disclose methods and/or apparatus with or without the use of chemicals.
Bartos, et al, U.S. Pat. No. 4,059,123 discloses a chemical cleaning method with a preservative. Likewise, Woodsen, et al, U.S. Pat. No. 4,808,235, and Sato, et al, U.S. Pat. No. 5,279,760 discloses different chemical cleaning solutions. Kaes, U.S. Pat. No. 5,002,078 discloses a chemical cleaning method for compressors that can be implemented off-line or on-line. Similarly, Amiran, U.S. Pat. No. 6,310,022 discloses a chemical cleaning composition to be used (off-line) for compressor cleaning while the compressor is being cranked.
Hodgens II, et al, U.S. Pat. Nos. 4,713,120 and 4,834,912 disclose a spray injection method and chemical compositions, respectively, for rinsing “baked-on” sand deposits from the compressor and turbine of aircraft type engines.
Hornak, et al, U.S. Pat. No. 4,196,020 discloses a wash spray nozzle apparatus for cleaning the compressor and turbine of a unit using a cleaning and rinsing method which also includes a preservative. Similarly, Butler, U.S. Pat. No. 6,394,108 discloses a specially fabricated flexible hose with nozzles on it which is inserted into the first several stages of an offline gas turbine compressor for compressor cleaning. McDermott, U.S. Pat. Nos. 5,011,540 and 5,273,395 disclose an apparatus and method utilizing a chemical solvent for cleaning compressors.
Hayward, et al, U.S. Pat. No. 6,073,637 discloses a water spray method for cleaning gas turbine compressors in which droplets of a cleaning fluid are sprayed into the compressor, comprising the steps of: spraying droplets of a substantially first uniform size into or onto the fluid path for a first period, and then spraying droplets of a substantially second uniform size into or onto the fluid path for a second period.
Three patents disclose methods of cleaning the hot gas path of a gas turbine. Langford, U.S. Pat. No. 4,065,322 discloses the use of coke particles to remove contaminants from the compressor and turbine sections, wherein the addition of coke particles is through the air stream. However, the main thrust of this method is to clean the compressor section, while the gas turbine is in operation with the hot gas path as a secondary aspect of the invention. Also Langford places no realistic size limit on these coke particles specifying an upper size of 3.35 mm, clearly too large to be safely used without causing damage to the gas turbine metallurgy. Gas turbine operating temperatures at the time Langford disclosed his coke were from 300 to perhaps higher cooler than modern gas turbines. The temperature required to “activate” the expandable coke of the present disclosure would not have been attainable at that time whereas they are presently. British Patent GB839762 (Ross) discloses a method for limiting the deposition of fuel oil ash on the blading and other parts of a gas turbine, wherein particles of carbonaceous material (graphite) are added to the fuel oil prior to combustion in a ratio of from ½% to 3% by weight. The carbonaceous materials can be used with or without a magnesium compound corrosion inhibitor, but the patent is silent as to the amount of magnesium. Koch, et al, U.S. Pat. No. 7,185,663 discloses the use of expandable graphite and/or molybdenum based particles and oil soluble corrosion inhibitors introduced directly into the combustion chamber of the gas turbine, into the fuel stream, water wash system, or the combustion air system (hot gas path). The linking of the use of the cleaning particles with a magnesium based corrosion inhibitor appears to be central to the disclosure.